Mystery surrounds Boris Johnson's whereabouts and his opposition to the planned third runway at Heathrow airport, with the foreign secretary expected to be out of the country for the crucial vote on the issue.
Chris Grayling, the transport secretary, said he was "cautiously optimistic" the expansion will be backed by MPs ahead of the vote in the Commons on Monday night, but said he had "no idea" where his cabinet colleague would be.
The government has said Mr Johnson will be abroad, but has so far declined to reveal where he will be, citing security considerations.
Mr Johnson once said he would lie down in front of bulldozers to prevent construction of the £14 billion runway.
He was challenged on Sunday by a Tory colleague to "put his money where his mouth is" and resign as foreign secretary over his opposition to the scheme.
Senior backbencher Sarah Wollaston said that Theresa May's decision to allow him to avoid her three-line whip in support of the Heathrow plan by going abroad "won't wash" and called on him to make a "principled decision" to stand down.
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“I have made clear my opposition since joining the Government, and I will continue to lobby colleagues from within government. Some of my critics have suggested that I should resign over the issue. No doubt they have my best interests at heart.
“But it is clear from what is likely to be a large majority of MPs who are in favour of a third runway that my resignation would have achieved absolutely nothing.
“Hillingdon council have been emphatic that they would rather have me in the cabinet and fighting for their cause on this and other issues.
“On election night I promised with John McDonnell, the Labour MP, to lie in front of the bulldozers. In view of the very considerable difficulties that still face the third runway — its cost and the appalling air and noise pollution entailed by the project — I believe it will be a very long time before we have to make good on that pledge; if indeed a 3rd runway ever comes about.”
Until now, the question was too crudely satirical to be worth asking. The only legally permissable response was a wintry cackle at the notion that, in such a narrow field, Boris Johnson could have a rival for the accolade.
Then up popped Boris Becker with his Central African Republic diplomatic passport, and what looked like a BoJo walkover finds itself in a tight fifth set....

Opinion: If Boris Johnson and Boris Becker battled over who would make a better foreign secretary, Becker would easily win
The IndependentEven if the former tennis champion's only reason for claiming diplomatic status is to pull a legal fast one and avoid going broke – does that make him a less credible diplomat than his fellow Boris?
George Osborne on Twitter
Twitter“Today’s @EveningStandard: sex prowler pleads guilty & Boris tells us why he’s missing Heathrow vote, as Lord Deighton explains why expansion is vital + @sebcoe on England’s football stars”"Unite, as a union, does support the expansion of Heathrow. I have my concerns, and I have expressed them and therefore will be voting against the expansion of Heathrow."

Will Boris Johnson lie down in front of the bulldozers? He was happy to lie down the side of a bus.
The IndependentIt took 50 years to come to the decision that Heathrow might have a third runway, and it took Zac Goldsmith almost as long to decide to resign over it
Minister resigns in protest over Heathrow expansion
The IndependentWalkout piles pressure on Boris Johnson – who is expected to be out of the country to avoid having to vote on MondayPlease allow the blog a moment to load.
Former whip Stephen Crabb told BBC Radio 4's Westminster Hour that Mr Johnson would "need to look his constituents in the eye and explain where he was on the night of the Heathrow vote".
Mr Grayling said: "I have no idea where Boris is, genuinely no idea where Boris is."
He added that "the Prime Minister has been very clear that there are people in the party who, for various reasons, have long held views about the airport and we are not going to whip those people into voting".
But Tory MP Greg Hands quit as international trade minister in order to vote against the plan because Conservatives were being whipped to support the expansion.
He said that he "wouldn't want to be abroad" for the vote, in a pointed reference to Mr Johnson's absence.
MPs will decide on Monday evening whether to approve the expansion of Europe's busiest airport, as more than 40 Labour members said they would go against party policy and support the Government's decision.
Labour is officially opposed to the expansion but Jeremy Corbyn has allowed MPs a free vote on a measure that is supported by trade unions but opposed by the shadow chancellor John McDonnell.
Mr Grayling told BBC Radio 4's Today programme he was hopeful the plan would be approved by MPs.
"I'm cautiously optimistic, it's never over until it's over and the vote actually happens but there is strong support across the political spectrum on this," he said.
"It's not usual for me to find myself campaigning on the same side as Len McCluskey of Unite but he is right in arguing that this is a project that can make a real difference to Britain, to jobs, to connections around the world and indeed to the whole UK because this is not simply a project for London and the south-east, the connections that we create through Heathrow benefit every part of the UK."
He added that it was "very clearly a private sector project" and "the taxpayer is not going to be paying for the expansion of Heathrow Airport".
Ahead of the vote, officials said the expansion of Heathrow would create 114,000 extra jobs in the area around the airport by 2030, with an extra 16 million long-haul seats by 2040.
It would represent the first full-length runway in the south east since the Second World War, the Department for Transport said.
Speaking to the Westminster Hour on Sunday, Totnes MP Dr Wollaston pointed out that Mr Hands had resigned and suggested Mr Johnson should follow.
She said: "I think this would be an opportunity for a colleague like Boris Johnson to actually put his money where his mouth is."
Mrs May last week confirmed he would miss the vote by being "the living embodiment of global Britain" abroad.
The Government has so far declined to say where Mr Johnson will be on security grounds although reports at the weekend suggested he may be somewhere in Africa.
The number of opposition MPs prepared to vote for Heathrow suggests the Commons vote on Monday night should pass with some ease.
Those who have signed the letter include many leadership critics, including Luciana Berger, John Mann, Mike Gapes and Wes Streeting, who argue it is right to back a scheme that will create jobs and growth.
Opponents have attacked the scheme on environmental, noise and financial grounds grounds, with Friends of the Earth saying it was "morally reprehensible" and would see the enlarged Heathrow emitting as much carbon as the whole of Portugal.
Paul McGuinness, chairman of the No 3rd Runway Coalition, said: "Heathrow expansion will be bad for London and bad for Britain."
Additional reporting by PA.